The present invention claims priority to Japanese Patent Document No. P2001-120615 filed on Apr. 19, 2001 herein incorporated by reference to the extent permitted by law.
The present invention relates to a vapor-phase growth method for a nitride semiconductor, which can be adapted to form a nitride semiconductor, such as a gallium nitride based compound semiconductor, on a base body by vapor-phase growth, and to a nitride semiconductor device, such as a nitride semiconductor light emitting diode, a nitride semiconductor laser, a suitable nitride semiconductor electron device formed by using the vapor-phase growth method or the like. More specifically, the present invention relates to a vapor-phase growth method for a nitride semiconductor, which can be adapted to form an anti-surfactant film on a base body and form a nitride semiconductor layer by crystal growth from an opening portion formed in the anti-surfactant film, and a nitride semiconductor device formed by using the vapor-phase growth method.
In recent years, nitride based III-V compound semiconductors, such as GaN, AlGaN and GaInN, have become a focus of attention. This is because such a semiconductor has a forbidden band width ranging from 1.8 eV to 6.2 eV, and therefore, the semiconductor theoretically allows realization of a light emitting device enabling emission of light ranging from red light to ultraviolet light.
In the case of fabricating a light emitting diode (LED) or a semiconductor laser by using a nitride based III-V compound semiconductor, it is required to form a structure in which multi-layers, such as a GaN layer, an AlGaN layer, a GaInN layer, and the like, are stacked such that a light emitting layer (active layer) is sandwiched between an n-type cladding layer and a p-type cladding layer. In some cases, such a light emitting diode or a semiconductor laser includes a light emitting layer having a quantum well structure composed of GaInN/GaN or GaInN/AlGaN.
A vapor-phase growth technique for a nitride semiconductor, such as a gallium nitride based compound semiconductor, can be problematic because it can be difficult to obtain a substrate that is lattice matched with a nitride semiconductor or a substrate having a low density of dislocations. To solve such a problem, there is known a technique of depositing a low temperature buffer layer made from AlN or AlxGa1-xN (0xe2x89xa6x less than 1) at a low temperature of 900xc2x0 C. or less on a surface of a substrate made from sapphire or the like, and then growing a gallium nitride based compound semiconductor thereon, thereby reducing dislocations due to lattice mismatching between the substrate and the compound semiconductor. Such a technique has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. By using such a technique, it is possible to obtain a gallium nitride based compound semiconductor with improved crystallinity and morphology.
Another technique of obtaining a high quality crystal structure at a low density of dislocations has been disclosed, for example, in Japanese Patent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This method involves depositing a first gallium nitride based compound semiconductor layer, forming a protective film made from a material capable of inhibiting growth of a gallium nitride based compound semiconductor, such as silicon oxide or silicon nitride, in such a manner as to selectively cover the first gallium nitride based compound semiconductor, and growing a second gallium nitride based compound semiconductor in an in-plane direction (lateral direction) from regions, not covered with the protective film, of the first gallium nitride based compound nitride layer, thereby preventing propagation of through-dislocations extending in the direction perpendicular to the interface of the substrate.
A further technique of reducing a density of through-dislocations has been disclosed, for example, in a document (MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)). This method involves growing a first gallium nitride based compound semiconductor, selectively removing the thus formed semiconductor film by using a reactive ion etching (hereinafter, referred to as xe2x80x9cRIExe2x80x9d) system, and selectively growing a second gallium nitride based compound semiconductor from the remaining crystal in the growth apparatus. According to this method, it is possible to obtain a crystal film having a density of dislocations, which is reduced to about 106/cm2, and hence to realize a high life semiconductor laser using the crystal film formed according to this method.
By the way, in the case of growing a gallium nitride based compound semiconductor layer by such a selective growth technique, a selective growth portion has a three-dimensional structure called xe2x80x9cfacetxe2x80x9d having a tilt plane which is a stable plane growing at a low crystal growth. For example, in the case of forming an underlying layer on a sapphire substrate with a C-plane of sapphire taken as a principal plane of the substrate, covering the surface of the underlying layer with a silicon oxide film as an anti-surfactant film (growth-inhibiting film), and selectively growing a gallium nitride layer from an opening portion provided in the silicon oxide film by supplying a source gas, crystal growth portion has a pyramid shape, for example, a hexagonal pyramid shape having a tilt plane covered with a crystal plane, such as an S-plane.
In this case, however, since a crystal growth rate is generally low at the tilt plane, a supplied source of a group III element is not deposited but migrated at the tilt plane. On the contrary, at a top portion of the pyramid shaped crystal growth layer, since the supplied amount of the source becomes excessively large, the crystal growth rate becomes significantly high. As a result, the top portion of the crystal growth layer contains a number of defects such as point defects, that is, has poor crystallinity. Further, crystal growth at the top portion of the crystal growth layer does not result in a smooth surface, and accordingly, in the case of fabricating a semiconductor device having an active layer or a pn-function particularly on the top portion, performances of the semiconductor device are significantly degraded.
A need, therefore, exists to provide improved nitride semiconductors that can be readily made and effectively used in a variety of applications.
An advantage of the present invention is to provide a vapor-phase growth method for a nitride semiconductor, which does not cause an excessive supply of a source at a top portion of a crystal growth layer formed by selective growth, thereby fabricating a nitride semiconductor device excellent in characteristics, such as a light emission characteristic.
Another advantage of the present invention is to provide an improved nitride semiconductor device fabricated by a vapor-phase growth method according to an embodiment of the present invention.
In an embodiment, the present invention provides a method of vapor-phase growth of a nitride semiconductor including the steps of supplying a first amount of a source material to a base body of the nitride semiconductor during a first time period; selectively growing a first portion of a nitride semiconductor layer on the base body during the first time period wherein the first portion is grown over a first area along a plane that is substantially parallel to a principle plane of the base body; supplying a second amount of the source material to the base body during a second time period; selectively growing a second portion of the nitride semiconductor layer on the base body during the second time period wherein the second portion is grown over a second area along the plane that is substantially parallel to the principle plane; and forming the nitride semiconductor wherein the second area of the selectively grown nitride layer is equal to or less than the first area of the selectively grown nitride layer and wherein the second amount of the source material is equal to or less than the first amount of the source material.
In another embodiment, the present invention provides a nitride semiconductor device a nitride semiconductor layer having a first portion and a second portion formed on a base body by selective growth wherein the first portion is selectively grown with a first amount of source material over a first area along a plane substantially parallel to a principal plane of the base body during a first time period, wherein the second portion is selectively grown with a second amount of source material over a second area along the plane substantially parallel to the principal plane during a second time period, and wherein the second area is equal to or less than the first area and the second amount of source material is equal to or less than the first amount of source material.
In yet another embodiment of the present invention, a method of vapor-phase growth of a nitride semiconductor is provided that includes the steps of selectively growing a first portion of a nitride semiconductor layer on a base body of the nitride semiconductor during a first time period at a first growth rate wherein the first portion is grown over a first area along a plane that is substantially parallel to a principle plane of the base body; selectively growing a second portion of the nitride semiconductor layer on the base body during a second time period at a second growth rate wherein the second portion is grown over a second area along the plane that is substantially parallel to the principle plane; and forming the nitride semiconductor wherein the second area of the selectively grown nitride layer is equal to or less than the first area of the selectively grown nitride layer and wherein the second growth rate is equal to or less than the first growth rate.
In still yet another embodiment, a nitride semiconductor device is provided that includes a nitride semiconductor layer having a first portion and a second portion formed on a base body by selective growth wherein the first portion is selectively grown at a first growth rate over a first area along a plane substantially parallel to a principal plane of the base body during a first time period, wherein the second portion is selectively grown at a second growth rate over a second area along the plane substantially parallel to the principal plane during a second time period, and wherein the second area is equal to or less than the first area and the second growth rate is equal to or less than the first growth rate.
In an further embodiment of the present invention, a method of vapor-phase growth for a nitride semiconductor is provided. The method includes forming a base body of the nitride semiconductor wherein the base body has a principal plane; and selectively growing a nitride semiconductor layer containing indium on the base body at a crystal growth rate ranging from about 1 xcexcm/h or less wherein the nitride semiconductor layer is grown along a plane that is perpendicular to a principal plane of the base body.
According to an embodiment of the present invention, there is provided a vapor-phase growth method for a nitride semiconductor, including the step of forming a nitride semiconductor layer on a base body by selective growth wherein an area of a plane, being nearly in parallel to a principal plane of the base body, of the nitride semiconductor layer grown by selective growth varies with respect to time S(t) (t: time), where an area S(t2) of the nitride semiconductor layer at a time t2 is equal to or smaller than an area S(t1) of the nitride semiconductor layer at a time t1 before the time t2, and a source supplied in amount X2 at the time t2 is equal to or smaller than a source supplied in amount X1 at the time t1.
In an embodiment, the nitride semiconductor layer can be a mixed crystal containing indium, or a mixed crystal layer containing indium can be grown on the nitride semiconductor layer which has been formed by selective growth. The vapor-phase growth method of the present invention can further include, in an embodiment, the step of forming an active layer on the plane, being nearly in parallel to the principal plane of the base body, of the nitride semiconductor layer, or forming an active layer on a tilt or slanted plane, being not in parallel to the principal plane of the base body, of the nitride semiconductor layer. The base body may be a gallium nitride based compound semiconductor substrate or a gallium nitride based compound semiconductor layer grown on a substrate made from a material different from a gallium nitride based compound semiconductor.
In selective growth, since an area of the nitride semiconductor layer grown over time tends to become small, the supply of a source to the opening portion tends to become large in proportion to an inverse of an opening ratio of a mask. In this case, since a side plane of the crystal growth layer is stable, the supply of the source of a group III element tends to become excessively large at the top portion. This can be prevented by adjusting the amount of source material supplied, depending on an area of the nitride semiconductor layer, particularly, the area of the top portion thereof according to an embodiment of the present invention. As a result, it is possible to improve the crystallinity and flatness or smoothness of the top portion of the nitride semiconductor layer.
According to another embodiment of the present invention, there is provided a vapor-phase growth method for a nitride semiconductor, including the step of forming a nitride semiconductor layer on a base body by selective growth; wherein an area of a plane, being nearly in parallel to a principal plane of the base body, of the nitride semiconductor layer grown by selective growth can vary with respect to time S(t) (t: time), where an area S(t2) of the nitride semiconductor layer at a time t2 is equal to or smaller than an area S(t1) of the nitride semiconductor layer at a time t1 before the time t2, and a crystal growth rate (V2) at the time t2 is equal to or smaller than a crystal growth rate (V1) at the time t1.
This method has an effect similar to that of the previously described vapor-phase growth method for a nitride semiconductor according to an embodiment. That is to say, in selective growth, an area of the nitride semiconductor layer grown over time tends to become small, and in this case, since a side plane of the crystal growth layer is stable, the crystal growth rate becomes significantly high as nearing the top portion of the nitride semiconductor layer, with a result that the top portion of the nitride semiconductor layer tends to contain a number of point defects. This can be prevented by adjusting the growth rate depending on an area of the top portion of the nitride semiconductor layer according to an embodiment of the present invention. As a result, it is possible to improve the crystallinity and flatness or smoothness, thereby effectively eliminates pitting on the top portion of the nitride semiconductor layer.
According to yet another embodiment of the present invention, there is provided a nitride semiconductor device including a nitride semiconductor layer formed on a base body by selective growth wherein an area of a plane, being nearly in parallel to a principal plane of the base body, of the nitride semiconductor layer grown by selective growth can vary with respect to time S(t) (t: time), where an area S(t2) of the nitride semiconductor layer at a time t2 is equal to or smaller than an area S(t1) of the nitride semiconductor layer at a time t1 before the time t2, and a source supplied amount X2 or a crystal growth rate V2 at the time t2 is equal to or smaller than a source supplied amount X1 or a crystal growth rate V1 at the time t1.
The nitride semiconductor device of the present invention can have a structure of a light emitting device, an electron device, or the like. The nitride semiconductor layer can be an active layer or an active layer may be grown on the nitride semiconductor layer which has been formed by selective growth. An active layer can be provided on the plane, being substantially parallel to the principal plane of the base body, of the nitride semiconductor layer, or provided on a tilt plane, being not in parallel to the principal plane of the base body, of the nitride semiconductor layer. The base body can be a gallium nitride based compound semiconductor substrate or a gallium nitride based compound semiconductor layer grown on a substrate made from a material different from a gallium nitride based compound semiconductor.
With the nitride semiconductor device, according to an embodiment of the present invention, the crystallinity and flatness of the top portion of a nitride semiconductor layer can be improved by adjusting the source supplied amount or crystal growth rate depending on the area of the nitride semiconductor layer, which is being selectively grown, particularly, the top portion thereof. The present invention can be widely applied to fabrication of a semiconductor device using selective growth. In particular, the present invention is useful for fabrication of a light emitting device, an electron device or other suitable device having a three-dimensional structure formed by selective growth. Further, since the flat nitride semiconductor layer with fewer defects can be formed, it may be preferably applied to an active layer or a layer on which an active layer is to be formed thereon.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.